1. Field of the Invention
This invention relates to the field of laser diodes having ultrahigh speed performance. More particularly, it relates to laser diodes cryogenically cooled to temperatures that provide increased modulation bandwidth.
2. Description of Related Art
Semiconductor laser diodes find use in numerous commercial applications, including optical fiber communication and data transmission. Typically, the laser diodes are operated at or near room temperature, although in certain applications, such as in space applications, the laser diodes may be exposed to cryogenic temperatures. For certain advanced applications, an ultrahigh speed laser diode, characterized by ultrahigh modulation bandwidth, having a high modulation depth with minimal signal distortion, is very desirable. Typical commercial laser diodes operated at room temperature have modulation bandwidths of less than a few GHz. The fastest commercially available high speed laser diodes have modulation bandwidths of about 12 GHz. Ultrahigh speed laser diodes with modulation bandwidths significantly exceeding 12 GHz will be an essential component for ultrahigh speed data transmission optical fiber links to meet the ever increasing demand for much higher data traffic in the national information infrastructure (also known as "Information Superhighway"). This invention provides an ultrahigh speed laser diode.
According to the invention, ultrahigh speed performance is achieved by cooling a laser diode to a range of low temperatures significantly beyond the common practice, and by selecting appropriate laser diode bias currents. By cooling the laser diode, the threshold current is also decreased, while the cooling also facilitates the use of higher bias currents by preventing the heating associated with the use of high bias currents in room temperature lasers.
A few workers have studied the effect of temperature on laser threshold current. They include J. O'Gorman et al.. "Temperature dependence of long wavelength semiconductor lasers," Appl. Phys. Lett. 60 1058-1060 (1992); N. K Dutta, et al. "Temperature characteristics of (InAs).sub.1 /(GaAs).sub.4 short-period lattices quantum well laser," Appl. Phys. Lett. 62 2018-2020 (1993); and L. E. Eng, et al., "Microampere threshold current operation of GaAs and strained InGaAs quantum well lasers at low temperatures (5 K)," Appl. Phys. Lett. 58 2752-2754 (1991). O'Gorman, Dutta and Eng did not report the effect of temperature on the modulation bandwidth of the lasers in their studies.
While little attention has been focused on the effect of cryogenic cooling on laser performance, much has been directed to improvement of their speed performance. Laser diodes operating at ultrahigh speeds, particularly those operating at 28 GHz or higher, offer opportunities for a number of applications not possible with slower laser diodes. Examples include satellite antenna remoting in the Ka or millimeter wave band, optical fiber delay lines for millimeter wave coherent applications, and long distance millimeter wave signal carrier transmission on optical fibers. For interface with the proposed 27-29 GHz commercial broadcast band, modulation bandwidths of at least 28 GHz would greatly simplify system design.
The highest modulation bandwidth for a semiconductor laser diode at communication wavelengths was reported by researchers at AT&T Bell Laboratories. P. A. Morton et al. "25 GHz bandwidth 1.55 .mu.M GaInAsP p-Doped Strained Multiquantum-Well Lasers," Electronics Letters 28 2456-2457 (1992) achieved a 25 GHz bandwidth in a multiquantum-well laser operated with a bias current of 180 mA. At a bias current of 40 mA, the modulation bandwidth was approximately 15 GHz. This high modulation bandwidth was achieved at the price of providing a very high bias current to the laser diode. A large bias current would not only be likely to result in unacceptably high signal distortion due to nonlinear light current characteristics, but would also generate excess heat resulting in premature failure of the laser diode.